The leading cause of low back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by damaged discs between the vertebrae, and low back pain is caused by collapse of the disc and by the adverse effects of bearing weight through a damaged unstable vertebral joint. One conventional method of managing this problem is to remove the problematic disc and fuse the adjacent vertebrae. Typically, the fusion is facilitated by filling the intevertebral disk space with autograft bone graft (such as bone chips) which contain matrix molecules and living cells such as osteoblasts which facilitate fusion.
U.S. Pat. No. 4,743,256 (xe2x80x9cBrantigan Ixe2x80x9d) discloses an improved surgical method for eliminating spinal back pain caused by ruptured or degenerated vertebral discs. In this procedure, the problematic disk is first removed, and the disc space created between adjacent vertebrae is filled with rigid inert implants, or xe2x80x9ccagesxe2x80x9d. As shown in FIG. 6 herein, the upper U and lower L surfaces of these cages C have large transverse pores P which facilitate bone ingrowth, and these pores lead to an inner void space IVS which houses bone graft (not shown) which facilitates the desired fusion. These cage surfaces are also shaped to fit within prepared endplates of the vertebrae to integrate the implant with the vertebrae and to provide a permanent load-bearing strut for maintaining the disc space. Brantigan I teaches that these cages typically consist of a homogeneous nonresorbable material such as carbon-reinforced polymers such as polyether ether ketone (PEEK) or polyether ketone ether ketone ketone (xe2x80x9cPEKEKKxe2x80x9d). Although these cages have demonstrated an ability to facilitate fusion, a sufficient fusion is sometime not achieved between the bone chips housed within the cage and the vertebral endplates. In particular, achieving a complete fusion in the middle portion of the cage has been particularly problematic.
Accordingly, there is a need for an interbody fusion device which facilitates fusion in the middle portion of the cage.
Cages similar to those disclosed in Brantigan I have been made from a laminated material comprising a plurality of polyaryl ether ketone (PEAK) layers. However, each of the layers therein is non-resorbable.
Published PCT Application No. WO 99/08627 (xe2x80x9cGresserxe2x80x9d) discloses a fully bioresorbable interbody fusion device, as well as homogeneous composite devices containing at least 25% resorbable materials. Although the bioresorbable nature of this device is an attractive feature, if fusion of the endplates through the disk space does not occur, the eventual resorption of the disclosed device may lead to collapse of the disk space.
U.S. Pat. No. 5,702,449 (xe2x80x9cMcKayxe2x80x9d) discloses a spinal implant comprising a cage made of a porous biocompatible material reinforced by an outer sleeve made of a second material which is relatively stronger under the compressive load of the spine than the biocompatible material. Although McKay teaches that any porous biocompatible material may be used as the support, only porous ceramics are particularly described. This porous biocompatible material appears to be a substitute for bone graft material. Under normal physiologic loads, the outer sleeve is intended to bear most of the load without bending or fracture, and to protect the more brittle ceramic therein, while the porous bioceramic support carries a portion of the initial load and slowly transfers it to the newly formed bone. The porosity of the bioceramic material may be up to about 700 microns. In one embodiment, McKay teaches that the height of the sleeve is less than the height of the porous biocompatible material, so as to permit the porous biocompatible material to contact the vertebral endplates. McKay discloses a manufacturing process which involves heat shrinking the sleeve around the bioceramic material.
Although the implant disclosed by McKay has utility, it also carries with it a number of disadvantages. First, even with the protective sleeve, the bioceramic is still inherently brittle and subject to catastrophic flaws. Second, the manufacturing process is relatively complicated. Third, the requirement that the sleeve fit around the bioceramic limits the design possibilities of the system. Lastly, there is no provision for the use of bone graft material.
Some fusion cages have been introduced which contain bioresorbable layers upon their inner surfaces. These layers are used as carriers for therapeutic drugs and do not extend beyond the outer surface of the cage. Accordingly, they provide very little structural contribution to the cage. For example, An abstract by Kandziora et al., discloses coating a conventional fusion cage with bioresorbable materials. See Kandziora et al. xe2x80x9cTGF-xcex2 and IGF-xcex2 Application by a Poly-(d,L)-Lactoide Coated Interbody Cage promotes Fusion In The Sheep Cervical Spinexe2x80x9d, Session 44, 47th Annual Meeting Orthopaedic Research Society, Feb. 25-28, 2001. These particular coatings comprised TGF-xcex2 and IGF-xcex2 disposed within a carrier matrix. The purpose of these coatings was to provide bone growth factors (TGF-xcex2 and IGF-xcex2) to the fusion site. Although there is no specific disclosure of the coating thickness in the Kandziora et al. abstract, such coatings typically have a thickness of 1-20 xcexcm, preferably 5-15 xcexcm.
The present invention relates to an intervertebral bone fusion device having a structural bioresorbable layer disposed upon the outer surface of a non-resorbable support. As the bioresorbable structural layer resorbs over time, the load upon the bone graft housed within the non-resorbable support increases. This invention provides the user with an interbody fusion device which offers many advantages not offered by the conventional technology.
In general, the rate at which a bone graft remodels into bone is determined in part by the loading upon that graft. In particular, the higher the loading upon the graft, the faster the graft remodels. Therefore, higher loadings of bone graft are normally desirable. Now referring to cross-sectional FIG. 7 depicting a conventional fusion site, when a conventional cage 91 filled with bone graft 93 is placed within the disc space, the bone graft is loaded by virtue of the upper 95 and lower 97 surfaces of the bone graft respectively contacting the upper 101 and lower 103 vertebral endplates.
However, the present inventors recognized that the vertebral endplates responsible for loading the graft contact not only the bone graft, but also the upper 105 and lower 107 surfaces of the cage 91 as well. Consequently, the loading upon the bone graft plug by the endplates is limited by virtue of this shared-contact condition.
Accordingly, the present inventors set out to construct a device wherein the loading produced by the vertebral endplates would not always be shared between the bone graft and the upper 105 and lower 107 surfaces of the cage, but rather would eventually be directed solely to the bone graft.
Preferably, the present invention relates to the disposition of a bioresorbable structural layer upon at least one of the upper or lower surfaces of a support in the form of a cage. The addition of the bioresorbable layer to the upper and/or lower surfaces of the cage creates a reservoir for additional bone graft extending from the upper or lower opening in the cage and allows the bone graft plug to be built to a height greater than the cage. Upon resorption of the bioresorbable layer, only the taller bone graft contacts the endplates. Accordingly, the entire load of the endplates is no longer shared between the bone graft and the cage, but is now accepted only by the bone graft. This increased loading promotes fusion.
Moreover, if the desired interbody fusion does not occur, the non-resorbed cage material remains in place in the disk space and functions as a normal cage, thereby preventing collapse of the disk space. Accordingly, the device of the present invention possesses the advantages of the conventional technologies but with enhanced osteogenic potential in its middle portion and without the risk of disk space collapse.
Therefore, now referring to FIG. 1a, and in accordance with the present invention, there is provided an interbody fusion device comprising:
a) a non-resorbable support 1 having an outer surface 11, the support having an osteoconductive pore 2 passing therethrough and opening onto at least two openings 15,17 on the outer surface 11, and
b) a first bioresorbable layer 31 disposed upon the outer surface 11 of the support,
wherein the bioresorbable layer has a thickness TH of at least 100 xcexcm.
Also in accordance with the present invention, there is provided an interbody fusion device comprising:
a) a non-resorbable support having an outer surface, the support having an osteoconductive pore passing therethrough and opening onto at least two openings on the outer surface, and
b) a first bioresorbable layer comprising a polymer, the layer contacting the support and extending beyond the outer surface of the support, wherein the first bioresorbable layer has a thickness TH of at least 100 xcexcm.
c)
Also in accordance with the present invention, there is provided an interbody fusion device comprising:
a) a non-resorbable support having an outer surface, the support having an osteoconductive pore passing therethrough and opening onto at least two openings on the outer surface, the pore defining an inner surface of the support, and
b) a first bioresorbable layer contacting the inner surface of the support and extending beyond the outer surface of the support,
wherein the contact of the bioresorbable layer upon the inner surface does not occlude at least two of the at least two openings at the outer surface, and wherein the first bioresorbable layer has a thickness TH of at least 100 xcexcm.